The invention relates to a mammalian pesticidal composition and method, in particular for the control of small underground dwelling mammals such as moles, shrews and voles.
Small, burrowing, underground-dwelling mammals, and moles in particular, can be a major agricultural pest, particularly to grass cropped areas. Their burrowing activities, or more specifically the earth mounds produced thereby, can result in serious damage to grass cutting equipment and contaminate resultant silage with soil to render it worthless. The creatures can also be pests in the domestic environment, for example in view of the damage done to garden lawns, golf course greens and the like.
The current generally recommended method for mole control is by strychnine poisoning. This technique is an effective and simple one, but it is an inhumane method of killing. Moreover, the high general toxicity of strychnine has led to its misuse against non-target species such as foxes, and necessitated a complex and cumbersome licensing procedure in many parts of the world. Other methods to control such pests, including gassing, trapping, and the use of sonic repellents have been tried, but with generally less efficacy than poison. There is therefore a general desire to develop a practical replacement poison to strychnine for pests of this type.
As a pesticidal agent which is active against mammalian pests, the applicants have found that volatile halogenated hydrocarbons which are generally those of low molecular weight (e.g. 3 or less carbon chain length) are particularly effective, if administered in a lethal dose. Such chemicals are known to have generally both an anaesthetic and in larger doses a toxic effect on some mammals. The use of one such compound, 1,2-dibromoethane, has been described for use as a rodent control agent when applied as a vapour to tunnels.
However, the volatility of these chemicals has rendered them susceptible to rapid dispersal into the environment so that maintaining concentrations at toxic dose levels is difficult. Furthermore, at low levels the chemicals are likely to have a repellent effect on the target. This means that halogenated hydrocarbons are suitably formulated to overcome these problems in order to provide a practical general purpose alternative to strychnine and similar poisons.
According to a first aspect of the invention there is provided the use of a composition comprising as pesticidal agent one or more low volatile halogenated hydrocarbons in microencapsulated form. In particular these compounds are microencapsulated in a gelatinous base.
Suitable gelatinous bases include gelatin, gelatin-gum acacia, gelatin-carrageenan, chitosan-carboxymethyl cellulose and chitosan pectin.
Microcapsules may be formed from these bases by a number of techniques including coacervation, spray drying, spray chilling, suspension separation processes, or centrifugal or other nozzle based extrusion processes.
Suitably the microcapsules comprise the above-described compounds in a coacervated gelatinous base.
The use of microcapsules loaded with the halogenated hydrocarbon overcomes the repellence and dispersal problems associated with the volatility of low molecular weight halogenated hydrocarbons. Additionally, since the first action of the halogenated hydrocarbon is an anaesthetic one, death by exposure to toxic dose levels is preceded by sedation, ataxia and anaesthesia, so that the use of this class of poison can be considered a more humane method of pest control than the use of strychnine and toxins of like action.
Gelatinous bases are generally hydrophilic and may therefore be prone to absorb environmental moisture in use. This can lead to rapid degradation of the microcapsule. The microcapsule is therefore preferably provided with an outer coating layer, which coating layer is of a material which is water resistant to render the microcapsule substantially impervious to moisture but is degradable in a mammalian alimentary system. The outer coating thus provides a moisture barrier to minimize environmental degradation and the leaching of toxins into the groundwater.
It will be understood that such coating must become permeable in the alimentary system if the poison is to take effect. This can conveniently be achieved by applying a coating which degrades in the acidic stomach environment of the target animal, one which degrades in the neutral or alkaline environment of the gut of the target animal, one which is digestible by the enzymes of the gut of the target animal, or one which melts at the body temperature of the target animal.
Suitable coating materials are known in the art. For instance, waxes are available which will melt at body temperature. Other possible coating materials include enteric polymers such as polyvinyl acetate phthalate, cellulose acetate phthalate, anionic copolymers of methacrylic acid and methyl methacrylate, hydroxy propyl methyl cellulose phthalate, and cellulose actetate trimelliate. Another possible coating material comprises a non-degradable coating material such as a polymer which has solid particles which are soluble in either the acid stomach conditions of the alkaline enviroment in the guts distributed throughout. Although this composite coating is complete when the microcapsules are formed, once the capsules come into contact with the conditions prevalent in an animals stomach or gut as appropriate, the solid material dissolves rendering the coating permeable. Suitable solid particles include calcium carbonate, barium carbonate and zinc carbonate. Using this stategy, coating materials such as shellac, fats, waxes, ethylcellulose and its derivatives, methyl methacrylate copolymers, polyethylene, polystyrene and polyvinylidene chloride may be used. A particular coating comprises calcium carbonate embedded in shellac.
Particularly suitable compositions for the pesticidal agent are chloro-, bromo- and iodoalkanes, in particular chloro-, bromo- and iodomethanes, with chloroform, bromoform and iodoform particularly preferred. Small mammals are found to exhibit a particular sensitivity to these agents. The microencapsulated composition produced thereby is thus to some degree targeted to the control of such pests.
Whilst the invention is not limited by any particular theory, it is believed that the toxicity of trihalomethane metabolites may be related to their reducing mechanisms. Free radical intermediates (*CHBr2) were found during aerobic and anaerobic incubation of bromoform with isolated rat hepatocytes. This radical was also detected in vitro incubation of bromoform with liver microsomes from mouse, chicken and turkey. The radical may lead to lipid peroxidation and destruction of cell membranes. The liver metabolism of insectivores appears to be such that they exhibit a particular and unexpected sensitivity to trihalomethanes, and so a pesticidal composition based upon a trihalomethane is thus to some degree targeted to the control of such pests. Thus the use of such compounds in pesticidal baits targeted to these pests forms a further aspect of the invention.
Table 1 below illustrates the significantly enhanced effectiveness of chloroform as an agent against moles, compared with rats and mice. An LD95 of chloroform for moles was determined as 200 mg/kg. The table compares this with LD50 data for rats and mice. In order to give a reasonable comparison, twice the LD50 for rats and mice is compared with the measured LD95 for moles. As the table shows, the effectiveness of chloroform on moles is in excess ten times that on rats and mice. It is established that effect of chloroform, bromoform and iodoform tends to follow a similar trend for any given species, so that a similar enhanced effectiveness can be extrapolated for other halomethanes.
Bromoform is particularly preferred and can be used alone as the sole pesticidal agent (save for incidental impurities) since it is less volatile than chloroform, barely soluble in water, and decomposed in air, a process that is speeded up by exposure to sunlight which will limit its tendency to contaminate the environment. In addition, approval for the use of chloroform based products is likely to be more difficult given, for example, its listing as a carcinogen by the EPA.
A microencapsulated composition in accordance with the invention, and in particular one in which bromoform is the principal or sole pesticidal agent, is particularly suited to the control of small, burrowing, underground-dwelling mammals such as moles since it is liable to decomposition in sunlight and therefore less likely to cause harm to surface dwelling small mammals such as hedgehogs, and is less susceptible than strychnine poisons to misuse as a surface poison against foxes and the like.
In a preferred aspect of the invention, the microcapsules containing the pesticidal composition are mixed with gum and added to food attractive to the target animal, such as chopped earthworms, to form a bait. The coating layer providing environmental protection of the gelatinous base remains important in this embodiment, since otherwise the hydrophilic gelatinous base will be degraded by the moist bait which could result in leakage of the pesticidal halogenated hydrocarbon. The resultant tainting of the chopped worms is likely to deter the target animal from taking the bait.
A further aspect of the invention provides for a method of controlling mammalian pests, and in particular a method of controlling small underground dwelling mammals such as moles, shrews and voles, comprising the use as a pesticidal agent of the above-mentioned compositions and baits. In particular the method uses one or more volatile halogenated hydrocarbons microencapsulated in a coacervated gelatinous base and preferably provided with a coating layer, which coating layer is of a material which is water resistant to render the microcapsule substantially impervious to moisture but is degradable in a mammalian alimentary system.
As above described, particularly suitable compositions for the pesticidal agent are chloro-, bromo- and iodoalkanes, in particular the halomethanes, with chloroform, bromoform and iodoform particularly preferred and bromoform most preferred.
A preferred aspect of the method comprises the use as a pesticidal bait of one or more microcapsules containing the pesticidal composition as above described which have first been mixed with gum and added to food selected to be attractive to the target animal, such as chopped earthworms.
A further aspect of the invention provides for a method of controlling small underground dwelling mammals such as moles, shrews and voles, comprising the use as a pesticidal agent of one or more haloalkanes. The method is particularly suited to control of insectivores, particular ground dwelling insectivores such as moles. Chloro, bromo- and iodoalkanes are preferred, and bromoform particularly so.
Details of particular pesticidal compositions and a suitable encapsulation materials and procedure, and a suitable bait incorporating the microencapsulated composition, will now be described by way of example.